I just looked up a Deskstar datasheet, and it specifies 5C to 60C operating, -40 to 70C non-operating. WD Caviar is 0-60C operating. Looks like it would be best to not freeze your hard drive.
It sounds like if the fans stop working, that would actually be what you want to keep the system warm...
The issue will be getting your data back off...........
Consider that:
1. These devices are mechanical.... simple physics states that materials expand and contract with temperature.
It means that as the temperature falls/rises. the tolerance of the mechanics may go out, you may record data on disk surfaces that have contracted/ with disk arms that are smaller, bearings that are tighter. When the mechanics return to the "correct" size it IS a problem.
2. Materials generally lag tracking the ambient temperature, which means that the drive will be colder than its surroundings when it get warmer, so a shit load of water may condense out on the electronics/inside the drives.